1. Field of the Invention
The present invention relates to an infrared transmitter. More specifically, the present invention relates to an infrared transmitter that can be stably used even with the increased number of channels used together by preventing generation of spurious noise and interference.
2. Description of the Related Art
Because infrared rays do not pass through walls, wireless communication utilizing the infrared rays provides excellent confidentiality. Infrared wireless microphones utilize the characteristic of the infrared rays to be free of signal interception from outside and to allow the same transmission channel to be used in two adjacent rooms without interfering. Thus, the infrared wireless microphones are used for many purposes.
An infrared transmitter, represented by the infrared wireless microphone, includes: an oscillator that supplies a carrier wave; a filter circuit that reduces a spurious signal of a transmission signal; an amplifier that amplifies an output from the filter circuit; and alight emitting diode (LED) that outputs an infrared signal.
The infrared transmitter employs an optical modulation system with which luminance of the LED is controlled according to the transmission signal. A single transmission channel uses a single carrier wave. Therefore, in an infrared transmitter with which a plurality of transmission channels can be used, the oscillator provides individual carrier waves as much as the number of the transmission channels.
A harmonic signal is a frequency component that is integer multiple of a fundamental frequency (here, carrier frequency). A radio signal emitted without eliminating the harmonic signal may cause interference between channels. Therefore, wireless transmission devices, including the infrared transmitter, have a filter circuit to eliminate the harmonic signal (spurious signal).
With a filter circuit with narrow passband, a plurality of carrier frequencies cannot be used. Thus, a multiple number of channels are difficult to be offered to the user. With a filter circuit with wide passband, the spurious signal cannot be eliminated. Thus, interference between adjacent channels is caused to degrade wireless communication.
Upon using together transmitters with which a plurality of channels can be used, the problem becomes more prominent. A relationship between the carrier frequency and the filter is described with reference to FIG. 2. In FIG. 2, the horizontal axis represents frequency; and the vertical axis represents output. FIG. 2 also depicts carrier frequencies f1 and f2 of an infrared transmitter. The use of the carrier frequency f1 is accompanied by generation of the spurious signal and an unintended frequency component is transmitted because the spurious signal has a certain signal level. Generally, the use of a carrier frequency f1 is accompanied by a plurality of spurious signals. For the convenience of explanation, FIG. 2 only depicts a second harmonic signal (f1×2).
A filter 1 is used to eliminate the harmonic signal (f1×2), which is a frequency component outside a necessary bandwidth of a signal to be transmitted from the infrared transmitter. Here, the filter 1 is a low-pass filter. Instead, the filter 1 may be a band pass filter. A characteristic of the filter 1 is depicted with a dotted line in FIG. 2. The spurious signal (f1×2) of the carrier frequency f1 can be eliminated with the filter 1. However, an infrared transmitter with which a plurality of signal channels can be used uses a plurality of carrier frequencies. For example, as shown in FIG. 2, the carrier frequency f2 is used together with the carrier frequency f1. The carrier frequency f2 is also accompanied by a harmonic signal (f2×2), which has a frequency higher than a cutoff frequency of the filter 1. Therefore, another filter that eliminates the harmonic signal (f2×2) is required.
To eliminate the harmonic signal (f2×2), a filter 2 is used that has a cutoff frequency higher than that of the filter 1. A characteristic of the filter 2 is described with a two-dot chain line in FIG. 2. If the cutoff frequency of the filter 2 is higher than that of the spurious signal (f1×2) of the carrier frequency f1, the spurious signal (f1×2) of the carrier frequency f1 cannot be eliminated therewith.
As a technique to solve the above problem, a transmitter is known with which a plurality of output signals can be used while reducing a spurious signal by switching between a plurality of filter circuits according to a frequency of an output signal (see, for example, Japanese Patent Application Publication No. 11-41120).
The technique disclosed in Japanese Patent Application Publication No. 11-41120 that reduces a spurious signal by appropriately switching between a plurality of filter circuits has following problems: when the number of transmission channels increases and bands between the channels become narrow, a plurality of filters are required having narrow band pass frequency corresponding thereto, thereby leading to an inconvenient larger sized circuit; and the use of a number of filter circuits by appropriately switching therebetween makes a stable operation difficult.